Normal flora refers to the microorganisms that are present even in state of normal health. Such microorganisms are always present in human bodies and their number far exceeds the number of body cells. A human body is composed of, on average, 10^13 cells. As compared to this the number of microorganism present in a human body at any time is about 10^14.

Normal flora perform many useful functions in the body such as:

Protection from highly pathogenic organisms.

Help in providing vitamins

Help in eliminating toxins

Certain type of normal flora can have harmful affects such as causing disease. There are other types of normal flora that neither help or harm.